This invention relates to fluid temperature responsive control devices, and particularly to temperature responsive flow control devices having a requirement to assume a particular flow control position in the event of failure of temperature responsive elements to function.
Thermostatic flow control devices are known to be positioned in a fluid flow path and to include a valve which at a predetermined high fluid temperature moves to a selected control position, as for example to close a shortened or bypass route compelling flow by way of a more circuitous route which may include a heat exchanger. Temperature sensitive elements in such devices are regarded as quite reliable but failure or impaired functional ability are not unknown. In some instances, failure or impaired capacity cannot be tolerated. In dealing with high temperature fluids, for example, flow in bypassing relation to a heat exchanger must be shut off or substantially reduced at a predetermined high temperature value to avoid mis-operation or hazards that may be associated with use of an over heated fluid.
Design of the valve assembly or of the system in which the valve functions may be compromised or made unduly complex as designers attempt to deal with this situation. Relatively elaborate steps have been taken to protect the valve against failure. Mechanisms have been introduced whereby response of the valve may be tested under conditions of simulated use as a part of a pre-use check. In some instances truly redundant capability is provided for by arranging two or more thermostatic devices in parallel so that a failure of one will find the other assuming or continuing performance of the desired function. In general, therefore, prior art efforts to cope with the problem of fail-safe operation in thermostatic valves and like devices have led to costly, relatively complex apparatus which often is of limited or uncertain utility.